Magnetic amplifier assisted LED constant current sink overhead voltage regulation

ABSTRACT

A power circuit include a plurality of LED strings, each LED string having multiple LEDs connected in series. A plurality of magnetic amplifiers, reset current sources, and a control circuit are used to drive each LED string with equal current and to independently regulate the amount of voltage supplied to each LED to maximize efficiency. One magnetic amplifier, one reset current source, and one current sink are dedicated to each LED string. The control circuit measures the voltage drop across each LED string and determines an amount of reset control current in response to the measured voltage drop. The reset control current is supplied by the reset current source to the magnetic amplifier dedicated to the regulated LED string. The amount of reset control current supplied to the magnetic amplifier dictates the amount of voltage supplied to the LED string.

FIELD OF THE INVENTION

The present invention relates to the field of power converters. Moreparticularly, the present invention relates to the field of regulatingvoltage applied to LEDs using magnetic amplifiers.

BACKGROUND OF THE INVENTION

Light emitting diodes (LEDs) are increasingly being used forbacklighting within display devices, such as liquid crystal display(LCD) televisions and monitors. Older technologies utilize CCFLs (coldcathode fluorescent lamps) for backlighting. The use of CCFLs hasnumerous drawbacks. For example, CCFLs contain mercury, which is a toxicmaterial. CCFLs also require very large voltages to operate, such asbetween 1 kV to 5 kV. As a result, providing the driving function ofCCFLs is difficult. The use of LEDs overcomes these drawbacks. LEDs donot contain mercury, and a single LED can operate at approximately 3.5volts, which eases the system design requirements. Further, LEDs have amuch longer operating life than CCFLs. The operating life of a CCFL isapproximately 20,000 hours, where the operating life is defined as theperiod of time over which the brightness of the CCFL is maintained abovea threshold level, such as one-half the original brightness. Thecomparable operating life of an LED is up to 50,000 hours.

In the context of backlighting applications, LEDs also suffer fromdrawbacks. Typically, an LCD television uses 200 or more LEDs, dependingon the display screen size. In one configuration, the LEDs are connectedin series. In this series configuration, the 200+LEDs times 3.5 voltsper LED still results in 700+V. In another configuration, the LEDs aredivided into multiple strings, each string includes a smaller subset ofLEDs. For example, a 45-50 inch LCD television may have 4-10 LED stings,each string having 50-150 LEDs. LEDs being essentially p-n junctiondiodes, do not share current equally when connected in parallel.Instead, the LEDs are connected in series as a string, and thus each LEDin the string shares the current. The strings then are drivenindividually. This string configuration enables operation at a lowervoltage. The string configuration is the route that manufactures havechosen.

A design challenge of the string based configuration is that in order toprovide uniform backlighting, each of the LED strings must be drivenwith equal current. This requires the use of regulated current sinks, orcurrent sources. A lowest cost approach is to provide a common anodevoltage to all LED strings and then regulate the current through eachcathode by using linear current sinks built around a transistor, such asa FET or BJT. However, manufactured LEDs do not have identical voltagedrops, and therefore the provided anode voltage must accommodate the LEDstring with the highest voltage drop. This results in increased powerloss across the current sinks corresponding to those LED strings havinglower voltage drops. Although individual LEDs or even LED strings can bebinned according to similar voltage drops, this comes at an added costto the LCD panel or television manufacturer. This still does notcompletely eliminate voltage drop differences in LEDs as all similarLEDs are still not identical and thus still results in some amount ofpower loss. A more efficient approach is to separately provide anindependent anode voltage to each LED string and then regulate thecurrent through the LED string using the linear current sink. The anodevoltage applied to a specific LED string is determined according to thevoltage drop of the specific LED string, not according to the LED stringwith the highest voltage drop. This results in optimized power lossacross the current sinks. However, providing independent anode voltageto each LED string requires more circuitry and is more costly.

FIG. 1 illustrates a block schematic diagram of an exemplaryconventional power circuit 10 used to power a plurality of LED strings.The power circuit 10 is an example of the second option described abovewhere an independent anode voltage is provided to each LED string. A PFCboost regulator 12 receives and converts an AC input to a full-waverectified sinewave current and regulates the current to 380 volts DC atthe output. A regulated safety isolated DC/DC converter 14 converts the380 volt signal to 24 volts, which is output to a boost regulator 16.The boost regulator 16 boosts the voltage from 24 volts to 100 volts. Insome cases, the converter 14 can be configured to output 100 volts so asto eliminate the boost regulator 16. Multiple boost regulators 18-22 areconnected in parallel to the boost regulator 16. There is one boostregulator for each LED string. For simplification, only one LED string26 is shown in FIG. 1, which is coupled to the boost regulator 18. Inimplementation, an additional LED string is coupled for each additionalboost regulator coupled in parallel to the boost regulator 18. The boostregulators 18-22 each boost the 100 volt input to 250 volts, therebysupplying the necessary voltage at point A for a current sink, such as atransistor 28, to be able to regulate the current for the correspondingLED string, in this case LED string 26.

The transistor 28 is coupled to the cathode of the LED string 26. Aseparate transistor is similarly coupled to each LED string. A linearcurrent sink control and boost feedback circuit 24 is separately coupledto the cathode of each LED string, such as the LED string 26, and toeach transistor, such as the transistor 28. The circuit 24 inconjunction with the current sinks function as current regulators forcontrolling the current provided to each LED string so that thebrightness of each LED string is uniform. Power loss through eachcurrent sink, such as the transistor 28, is minimized by sensing thevoltage at point A and providing appropriate feedback to thecorresponding boost regulator, such as boost regulator 18, to regulatethe voltage applied to the anode of each LED string, such as the anodeof the LED string 26. There is a minimum compliance voltage that must bemaintained at point A in order for the transistor 28 to function as aneffective current sink and regulate the current through the LED string26. The boost regulator 18 adjusts the output boost voltage applied tothe anode of the LED string 26 such that the output voltage minus thevoltage drop across the LED string 26 is equal to the minimum compliancevoltage necessary at point A. A boost voltage that results in a voltageat point A greater than the minimum compliance voltage leads to powerloss across the transistor 28. A boost voltage that results in a voltageat point A less than the minimum compliance voltage does not enable thetransistor 28 to function as a current sink and therefore does not leadto proper current regulation through the LED string 26.

The minimum compliance voltage at point A can be set to any level, suchas 1V, 10V, or 15V. The voltage level is set according to the transistor28 and the actual voltage drop across the LED string 26. For example,the power circuit manufacturer receives the LEDs from a vendor, but theexact voltage drop for each LED string is not known because thespecifications for each LED may vary. In order to accommodate thisunknown voltage the power circuit manufacturer has two choices. One, theoutput of the boost regulator 18 can be fixed to a set output voltagelevel and the transistor 28 is allowed to absorb the extra voltage,which is the output voltage of the boost regulator 18 minus the voltagedrop across the LED string 26. The transistor 28 burns this excessvoltage as power by supplying the absorbed voltage as current to thetransistor 28. This option presents an inefficient situation because inany given batch of LEDs, each LED string may have a voltage drop that isless than the highest possible voltage drop designed into the circuit.This may result in the total power burned through all the transistorscoupled to the LED strings being upwards of a few watts. A way tocircumvent this inefficiency is to measure the drain voltage of thetransistor 28 (at point A) and supply that measurement as feedback tothe boost regulator 18, which then adjusts the output boost voltageaccordingly. This feedback method essentially regulates the voltage atpoint A to the minimum compliance voltage required for the transistor 28to function as a current sink with the desired amount of current.

In the configuration of FIG. 1, the boost regulator 18 is conductive.The boost regulator 18 optimally adjusts the overhead voltage of thetransistor 28 to the value that is necessary to maintain regulation ofthe current through the LED string 26, but not more than that. This isan optimum situation from an efficiency perspective. However, the powercircuit 10 then requires one boost circuit (boost regulator) for eachLED string plus a corresponding linear current sink controller circuit24, one control loop coupled to one of the corresponding boostregulators 18-22. This makes for an expensive and complex system.

FIG. 2 illustrates a block schematic diagram of another exemplaryconventional power circuit 40 used to power a plurality of LED strings.The power circuit 40 is an example of the first option described above,the lowest cost approach where a common anode voltage is provided to allLED strings. The power circuit 40 of FIG. 2 differs from the powercircuit 10 of FIG. 1 in that the power circuit 40 has a single boostregulator 48 (instead of the multiple boost regulators 18-22) and asingle boost feedback. The output of the boost regulator 48 is providedas a common boost voltage to the anode of each LED string. A currentsink is coupled to each LED string, such as a transistor 54 coupled to aLED string 52. A linear current sink controller 50 includes separatecontrol loops for each LED string and current sink pair. The separatecontrol loops provide the single feedback to the boost regulator 48. Theefficiency of the power circuit 40 of FIG. 2 is not as high as theefficiency of the power circuit 10 of FIG. 1. The reason is that theboost voltage is separately provided to each LED string in FIG. 1, andthat each LED string current in FIG. 1 is being regulated by acorresponding linear current sink transistor, such as transistor 28, anda corresponding control circuit. Despite the simplicity and lower costof the power circuit 40 in FIG. 2 compared to the power circuit 10 ofFIG. 1, this benefit comes at the expense of lower efficiency becausethe single boost regulator output voltage must be regulated to levelthat accommodates the highest LED string voltage drop. In eitherconfiguration, the power circuits do not fully eliminate power loss.

SUMMARY OF THE INVENTION

Embodiments of a power circuit include a plurality of LED strings, eachLED string having multiple LEDs connected in series. A plurality ofmagnetic amplifiers, reset current sources, and a control circuit areused to drive each LED string with equal current and to independentlyregulate the amount of voltage supplied to each LED to maximizeefficiency. One magnetic amplifier, one reset current source, and onecurrent sink are dedicated to each LED string. A power conversioncircuit provides isolated power to the plurality of LED strings via theplurality of magnetic amplifiers. The control circuit measures thevoltage drop across each LED string and determines an amount of resetcontrol current in response to the measured voltage drop. The resetcontrol current is supplied by the reset current source to the magneticamplifier dedicated to the regulated LED string. The amount of resetcontrol current supplied to the magnetic amplifier dictates the amountof voltage supplied to the LED string. The control circuit includes aseparate control loop coupled to each current sink.

In one aspect, a power circuit includes a magnetic amplifier; a lightemitting diode string coupled to an output of the magnetic amplifier,wherein the light emitting diode string includes a plurality of lightemitting diodes coupled in series; and a control circuit coupled to themagnetic amplifier and the light emitting diode string, wherein themagnetic amplifier and the control circuit are configured to regulate acurrent through the LED string. In some embodiments, the power circuitalso includes a current sink transistor coupled to the LED string and tothe control circuit, wherein the magnetic amplifier and the controlcircuit are configured to maintain a compliance voltage of the currentsink transistor. The control circuit includes an integrated control loopconfigured to regulate the current through the LED string and to controla reset current supplied to the magnetic amplifier. In someconfigurations, the power circuit includes a plurality of magneticamplifiers and a plurality of light emitting diode strings, one magneticamplifier coupled to one light emitting diode string to form a magneticamplifier and light emitting diode string pair, wherein the controlcircuit includes a plurality of control loops, one control loop coupledto one magnetic amplifier and light emitting diode string pair.

In another aspect, a power circuit is disclosed that includes a powerconversion module including a transformer, a primary winding of thetransformer coupled to a power source; a magnetic amplifier coupled to asecondary winding of the transformer; a light emitting diode stringcoupled to an output of the magnetic amplifier, wherein the lightemitting diode string includes a plurality of light emitting diodescoupled in series; a reset current source coupled to the output of themagnetic amplifier; a current sink coupled to an output of the lightemitting diode string; and a control circuit coupled to the resetcurrent source, the light emitting diode string, and to the currentsink. In some embodiments, the power conversion module is configured forpower factor correction, DC-to-DC conversion and power isolation. Insome embodiments, the magnetic amplifier has a saturable magnetic coreand one or more windings around at least a portion of the core. In oneconfiguration, the magnetic amplifier has one winding used to transmitpower to the light emitting diode string and to receive a reset currentfrom the reset current source. In another configuration, the magneticamplifier has two windings, a first winding is configured to transmitpower to the light emitting diode string and a second winding isconfigured to receive a reset current from the reset current source. Insome embodiments, the reset current source is a transistor. In someembodiments, the current sink is a transistor. A B-H curve of themagnetic amplifier exhibits hysteresis where the magnetic amplifiermaintains a flux density with zero current passing through substantiallyequal to a saturation flux density level. The magnetic amplifier can beconfigured to pass a power current to the light emitting diode stringwhile in a saturated state and during an on-time of a power supplysignal input to the magnetic amplifier. The control circuit can beconfigured to provide a control signal to the reset current source suchthat the reset current source forces a reset current through themagnetic amplifier during an off-time of the power supply signal,further wherein the magnetic amplifier changes from the saturated stateto a non-saturated state due to the reset current. If changed to thenon-saturated state during the off-time, the power current is delayedfrom passing through the magnetic amplifier during the on-time of thepower supply signal until the magnetic amplifier changes from thenon-saturated state to the saturated state during the on-time of thepower supply signal. An amount of the delay is a function of an amountof the control current. An amount of power provided to the lightemitting diode string is an inverse function of the amount of the resetcurrent. In some embodiments, the reset current source, the currentsink, and the control circuit are integrated on a same integratedcircuit. The control circuit is configured to control the current sinkso as to maintain a constant current through the light emitting diodestring.

In yet another aspect, the power circuit is configured for a pluralityof light emitting diode strings that are individually controlled. Inthis configuration, the power circuit includes a power conversion moduleincluding a transformer, a primary winding of the transformer coupled toa power source; a plurality of magnetic amplifiers coupled in parallelto a secondary winding of the transformer; a plurality of light emittingdiode strings, one light emitting diode string coupled to an output ofone of the plurality of magnetic amplifiers, wherein each light emittingdiode string includes a plurality of light emitting diodes coupled inseries; a plurality of reset current sources, one reset current sourcecoupled to the output of one of the plurality of magnetic amplifiers; aplurality of current sinks, one current sink coupled to an output of oneof the plurality of light emitting diode strings; and a control circuitcoupled to the plurality of reset current sources, to the plurality oflight emitting diode strings, and to the plurality of current sinks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block schematic diagram of an exemplaryconventional power circuit used to power a plurality of LED strings.

FIG. 2 illustrates a block schematic diagram of another exemplaryconventional power circuit used to power a plurality of LED strings.

FIG. 3 illustrates a conceptual block diagram of a power circuitaccording to a first embodiment.

FIG. 4 illustrates an exemplary B-H curve of a magnetic amplifier.

FIG. 5 illustrates an exemplary voltage curve of the voltage Vs acrossthe secondary winding of the transformer in FIG. 3.

FIG. 6 illustrates the voltage across the magnetic amplifier if themagnetic amplifier is maintained in a saturated state, the magneticamplifier receives the voltage waveform of FIG. 5.

FIG. 7 illustrates the current I_(L) passing through the magneticamplifier if the magnetic amplifier is maintained in a saturated state,the magnetic amplifier receives the voltage waveform of FIG. 5.

FIG. 8 illustrates the voltage across the magnetic amplifier if a resetcurrent is applied to the magnetic amplifier during off-time, themagnetic amplifier receives the voltage waveform of FIG. 5.

FIG. 9 illustrates the current I_(L) passing through the magneticamplifier if the reset current is applied to the magnetic amplifierduring off-time, the magnetic amplifier receives the voltage waveform ofFIG. 5.

FIG. 10 illustrates the B-H curve corresponding to the waveforms shownin FIGS. 8 and 9.

FIG. 11 illustrates an exemplary configuration of a magnetic amplifier.

FIG. 12 illustrates an exemplary configuration of the magnetic amplifierof FIG. 9 coupled to a reset current source.

FIG. 13 illustrates a conceptual block diagram of a power circuitaccording to a second embodiment.

Embodiments of the power circuit are described relative to the severalviews of the drawings. Where appropriate and only where identicalelements are disclosed and shown in more than one drawing, the samereference numeral will be used to represent such identical elements.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present application are directed to a power circuit.Those of ordinary skill in the art will realize that the followingdetailed description of the power circuit is illustrative only and isnot intended to be in any way limiting. Other embodiments of the powercircuit will readily suggest themselves to such skilled persons havingthe benefit of this disclosure.

Reference will now be made in detail to implementations of the powercircuit as illustrated in the accompanying drawings. The same referenceindicators will be used throughout the drawings and the followingdetailed description to refer to the same or like parts. In the interestof clarity, not all of the routine features of the implementationsdescribed herein are shown and described. It will, of course, beappreciated that in the development of any such actual implementation,numerous implementation-specific decisions must be made in order toachieve the developer's specific goals, such as compliance withapplication and business related constraints, and that these specificgoals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

Embodiments of a power circuit include a plurality of LED strings, eachLED string having multiple LEDs connected in series. A plurality ofmagnetic amplifiers, reset current sources, and a control circuit areused to drive each LED string with equal current and to independentlyregulate the amount of voltage supplied to each LED to maximizeefficiency.

FIG. 3 illustrates a conceptual block diagram of a power circuitaccording to a first embodiment. The power circuit 100 includes a powerconversion module 102, a plurality of magnetic amplifiers LM1-LMn, aplurality of LED strings 114-120, a plurality of transistors Q1-QN+1,and a linear current sink control circuit 122. For simplicity, only thecircuit configuration and operation related to the LED string 114 isdescribed below. The configuration and operation of the circuitrycoupled to the remaining LED strings 116, 118, 120 are the same as thatdescribed related to the LED string 114.

One magnetic amplifier is coupled to one LED string, for example the LEDstring 114 is coupled to the output of the magnetic amplifier LM1. Arectifier circuit including a diode D2, a freewheeling diode D3, afilter inductor and filter L1, and a filter capacitor C1 is coupledbetween the magnetic amplifier LM1 and the LED string 114. In someembodiments, the diode D3 and the filter inductor and filter L1 may notbe necessary. The transistor Q1 functions as a reset current source toprovide a reset current to the magnetic amplifier LM1. It is understoodthat alternative types of current sources can be used. In someembodiments, the transistor Q1 is a bipolar junction transistor (BJT).In other embodiments, the transistor Q1 can be any other type ofconventional transistor. A diode D1 is coupled between the transistor Q1and the magnetic amplifier LM1. The diode D1 provides reverse voltageblockage. The transistor Q2 functions as a current sink, also referredto in this case as a current regulator, for the LED string 114. In someembodiments, the transistor Q2 is a metal-oxide-semiconductorfield-effect transistor (MOSFET). In other embodiments, the transistorQ2 can be any other type of conventional transistor. The circuitoperates such that the voltage at point A (drain of the transistor Q2)is at an optimal level for each LED string. The optimal voltage level atpoint A is high enough to maintain the transistor Q2 in the linearregion so as to act as a good current sink, but low enough so as tominimize, if not eliminate, power loss across the transistor Q2. Thisoptimal level is referred to as the minimum compliance voltage. Thedrain, gate, and source of the transistor Q2 are all coupled to thecontrol circuit 122. The control circuit 122 measures the drain voltageat point A to determine a reset current to be provided by the transistorQ1 to the magnetic amplifier LM1, as will be explained in greater detailbelow. A control signal is sent from the control circuit 122 as a gatevoltage to the transistor Q1, which in turn provides the reset current.Independent reset currents are provided by each of the other transistorsQ3, Q5, and QN so as to independently regulate each LED string voltage.

In the power circuit of FIG. 3, conventional boost regulators on thesecondary winding side of a power conversion module are replaced bymagnetic amplifiers and controlled current sources used to regulate thecurrent provided to each LED string. The control circuit 122 inconjunction with the current sinks function as current regulators forcontrolling the current provided to each LED string so that thebrightness of each LED string is uniform. Power loss through eachcurrent sink, such as the transistor Q2, is minimized by sensing thevoltage at point A and providing appropriate reset current to thecorresponding magnetic amplifier, such as the magnetic amplifier LM1, toregulate the voltage applied to the anode of each LED string, such asthe anode of the LED string 114. There is a minimum compliance voltagethat must be maintained at point A in order for the transistor Q2 tofunction as an effective current sink and regulate the current throughthe LED string 114. The reset current is set to adjust the outputvoltage of the magnetic amplifier LM1 such that the voltage at point Ais equal to the minimum compliance voltage. The minimum compliancevoltage at point A can be set to any level, such as 1V, 10V, or 15V.

The control circuit 122 includes separate control loops for each LEDstring. A function of each control loop is to maintain a constantcurrent through the corresponding LED string. The control loop measuresthe voltage at point B and compares this measured voltage to a referencevoltage. The reference voltage can be a fixed value set at the time ofmanufacture, or the reference voltage can be externally provided as asystem command. In general, the reference voltage value is applicationspecific. The difference between the measured voltage and the referencevoltage is used to determine a gate voltage supplied to the current sinktransistor Q2, which regulates a constant current through the LED string114. The brightness of the LED string 114 can be changed, not bychanging the current through the LED string, but instead by adjustingthe duty cycle of the current. The brightness level is received as theDIM input received by the control circuit 122.

The power conversion module 102 may combine a power factor correction(PFC) boost regulator and an isolated DC-to-DC power converter, therebyresulting in a single-stage PFC and DC-to-DC power conversion withisolation. The power conversion module 102 rectifies an input AC voltageand provides safety isolation through a transformer 104, which is arequirement of consumer electronic devices connected to an AC line.Typically, the switching frequency applied to the transformer 104 is inthe range of 100-250 kHz. Frequencies greater than this range sufferfrom losses incurred as a result of switching at higher voltages, suchas 300-400 volts. A power waveform is output at the secondary winding ofthe transformer 104, typically this waveform is a square wave.

In the conventional power circuits of FIGS. 1 and 2, the power waveformoutput from the isolating DC-to-DC converter are passed through a diodenetwork to rectify the waveform and through a filter to smooth thewaveform to a desired DC level, such as the 24 volt DC at the output ofthe DC-to-DC converter 14/44 in FIGS. 1 and 2. In the power circuit 100of FIG. 3, a rectifying diode network and filter are used after theplurality of magnetic amplifiers LM1-LMn. One magnetic amplifier iscoupled between the secondary winding of the transformer 104 and one ofthe plurality of LED strings via a corresponding rectifier filternetwork. As shown in FIG. 3, a magnetic amplifier LM1 is coupled betweenthe transformer 104 and the LED string 114 via a corresponding rectifierfilter network. Similarly, a magnetic amplifier LM2 is coupled betweenthe transformer 104 and the LED string 116 via a corresponding rectifierfilter network, a magnetic amplifier LM3 is coupled between thetransformer 104 and the LED string 118 via a corresponding rectifierfilter network, and a magnetic amplifier LMn is coupled between thetransformer 104 and the LED string 120 via a corresponding rectifierfilter network.

The magnetic amplifier functions substantially as a switch that can becontrolled to regulate the amount of voltage provided to the coupled LEDstring. The characteristics of the magnetic amplifier are used toaccomplish this switching and voltage regulating functionality.

Magnetic materials can be described by a B-H curve, where B is measureof the magnetic flux density and H is a measure of the magnetic fieldstrength. Where an inductor has no core, for example a looped wire, theB-H curve is a straight line. The slope of the line is the permeabilityof vacuum, μ₀. The field strength H is proportional to the number ofwindings N in the looped wire times the current I passing through thelooped wire. As current I passing through the wire is increased, whichcorresponds to an increase in the field strength H, the flux density Bincreases linearly according to the permeability μ₀.

A magnetic amplifier is an electromagnetic device for amplifyingelectrical signals. A magnetic amplifier is an inductor wound about acore made of a special magnetic material that has a substantiallyrectangular B-H curve and high remaining flux even after the excitingcurrent is turned off. A magnetic amplifier may visually resemble aninductor or even a transformer wound in a toroidal core or other coreshape of closed magnetic path, but the operating principle is quitedifferent. The magnetic amplifier makes use of magnetic saturation ofthe core. The magnetic amplifier includes core materials designed tohave a specific B-H curve shape that is highly rectangular, in contrastto the slowly-tapering B-H curve of softly saturating core materialsthat are often used in normal inductors or transformers.

FIG. 4 illustrates an exemplary B-H curve of a magnetic amplifier. TheB-H curve of the magnetic amplifier shows hysteresis. The relationshipbetween the field strength H and the flux density B is not linear in amagnetic amplifier. If the relationship between the two is plotted forincreasing levels of field strength H, the B-H curve reaches a pointwhere further increases in field strength H results in no further changein flux density B. This condition is called magnetic saturation.Referring to FIG. 4, as the current, which flows through the inductorwindings of the magnetic amplifier, increases (positive H), the fluxdensity B increases according to the right-hand portion of the B-Hcurve. The slope of this portion of the B-H curve, for example at pointB, is equal to the core permeability μ_(m) of the magnetic amplifier.The flux density B reaches a point A where as the current continues toincrease, the flux density B does not increase. In actuality, thehorizontally flat portion of the B-H curve is not completely horizontal,having a positive slope equal to the vacuum permeability μ₀. The corepermeability μ_(m) is much greater than the vacuum permeability μ₀, asshown by the difference in the positive slope at point B versus therelatively flat slope of the nearly horizontal line passing throughpoint A.

When current is removed from the magnetic amplifier, the flux density Bdoes not fall to zero. Instead, the magnetic amplifier remains saturatedwith a flux density B_(S), at point C. It is shown in FIG. 4 that at H=0(corresponding to zero current passing through the magnetic amplifier),the flux density B is not zero, but has a non-zero value B_(S). The fluxdensity B_(S) is very close to the flux density B when full current isapplied, such as at point A. The difference in value resulting from theslight slope μ₀ of the nearly horizontal portion of the B-H curve. Thenon-zero value of the flux density B when H=0 is due to the magneticcharacteristics of the magnetic amplifier's core material. To reduce theflux density from B_(S), a negative current is applied to the magneticamplifier.

The inductance L of the magnetic amplifier is proportional to thepermeability μ. The inductance L is an indicator of the impedance of thepower circuit. The higher the inductance L, the higher the circuitimpedance. The lower the inductance L, the lower the circuit impedance.When the magnetic amplified is saturated, which corresponds to thenearly horizontally flat portion of the B-H curve, the power circuit hasa low impedance because the permeability of the magnetic amplifier insaturation is very small (inductance L is proportional to thepermeability μ). When the magnetic amplifier is not saturated, whichcorresponds to the B-H curve where the slope equals the corepermeability μ_(m), the inductance of the magnetic amplifier is muchhigher, which results in a higher circuit impedance. It is thisimpedance characteristic that is applied to the power circuit of FIG. 3.When the current through the magnetic amplifier is zero, the fluxdensity does not go to zero but instead remains at the value B_(s), thiscorresponds to point C on the B-H curve. If a signal, such as a currentI_(L) input from the secondary winding of the transformer 104 (FIG. 3),is passed through the magnetic amplifier while the magnetic amplifier isin the saturated state at point C on the B-H curve, then the signalexperiences low impedance because the inductance L of the magneticamplifier at point C is low. If a current source is connected to themagnetic amplifier, such as the transistor Q1 shown in FIG. 3, so as toforce negative current through the magnetic amplifier, then the magneticamplifier is forced out of the saturated stated, as shown in section Dof FIG. 4. Application of the negative current reduces the flux densityB from the saturated level B_(s) toward zero (along section D of the B-Hcurve). While the magnetic amplifier operates within section D of theB-H curve, any signal (current I_(L)) passing through the magneticamplifier experiences a much higher impedance because the inductance Lof the magnetic amplifier within section D is high for the amount oftime it takes the applied power pulse to saturate the core. Currentflows through the magnetic amplifier while saturated (low impedance),but current is substantially smaller through the magnetic amplifierwhile not saturated (high impedance). In this manner, the magneticamplifier can function substantially as a switch controlled by the resetcurrent supplied by the reset current source. The reset current sourceconnected to the magnetic amplifier forces reset current through themagnetic amplifier in the opposite direction as the current I_(L). Theamount of this rest current is controlled by the linear current sinkcontrol circuit. Adjusting the amount of reset current modulates theamount of time that high impedance is presented by the magneticamplifier to the power circuit, and therefore regulates the amount ofcurrent I_(L) that passes through the magnetic amplifier by modulatingthe effective duty cycle.

FIG. 5 illustrates an exemplary voltage curve of the voltage Vs acrossthe secondary winding of the transformer 104 in FIG. 3. Power istransferred from the power conversion module 102 to the circuitconnected to the secondary winding of the transformer 104. The powertransfer occurs during the positive portion of the voltage Vs shown inFIG. 5. The amount of current I_(L) that flows through the magneticamplifier is determined by the voltage Vs applied to the magneticamplifier and also by the amount of magnetic amplifier impedance. Forexample, if the magnetic amplifier is in a saturated state (point A or Cin FIG. 4), then the magnetic amplifier presents essentially zeroimpedance to the power circuit because the B-H curve is essentiallyhorizontal. During the positive portion of the voltage Vs, referred toas “on-time”, and while the magnetic amplifier is in the saturatedstate, the current I_(L) flowing through the magnetic amplifier passesthrough the magnetic amplifier unchanged because of the zero impedance.FIG. 6 illustrates the voltage across the magnetic amplifier if themagnetic amplifier is maintained in a saturated state, the magneticamplifier receives the voltage waveform of FIG. 5. FIG. 7 illustratesthe current I_(L) passing through the magnetic amplifier if the magneticamplifier is maintained in a saturated state, the magnetic amplifierreceives the voltage waveform of FIG. 5.

If reset current is forced through the magnetic amplifier during thenegative portion of the voltage Vs, referred to as “off-time”, the stateof the magnetic amplifier is changed according to the B-H curve. Forexample as reset current is applied (negative current relative to themagnetic amplifier and the magnetic amplifier B-H curve), the fluxdensity B of the magnetic amplifier moves into section D (FIG. 4) of theB-H curve, which corresponds to high impedance. The more reset currentthat is applied, the further down section D of the B-H curve themagnetic amplifier moves. For the next positive portion of the voltageVs, a finite amount of time is required for the magnetic amplifier tomove back up the B-H curve and reach saturation. FIG. 8 illustrates thevoltage across the magnetic amplifier if a reset current is applied tothe magnetic amplifier during off-time, the magnetic amplifier receivesthe voltage waveform of FIG. 5. FIG. 9 illustrates the current I_(L)passing through the magnetic amplifier if the reset current is appliedto the magnetic amplifier during off-time, the magnetic amplifierreceives the voltage waveform of FIG. 5. FIG. 10 illustrates the B-Hcurve corresponding to the waveforms shown in FIGS. 8 and 9. Period Icorresponds to the magnetic amplifier returning to saturation duringon-time after a reset current has been applied during off-time. Untilsaturation is reached, the magnetic amplifier is in the high impedancestate. While in a high impedance state, current does not flow throughthe magnetic amplifier, as shown in period I of FIG. 9. Once saturationis reached, period II, the magnetic amplifier is in the zero impedancestate and the current I_(L) passes through. At period III, the voltageVs applied to the magnetic amplifier changes from on-time to off-time.During off-time, reset current is applied to the magnetic amplifier,thereby moving the magnetic amplifier down the B-H curve, period IV.During the next on-time cycle, the magnetic amplifier again moves backup the B-H curve, period I.

The amount of time required for the magnetic amplifier to reachsaturation during the on-time (positive portion of the voltage Vs) isdependent on the amount of reset current applied during the off-time(negative portion of the voltage Vs). The amount of reset currentapplied dictates how far the magnetic amplifier moves down the B-Hcurve. As shown in FIG. 10, the deeper B-H curve (dotted line)corresponds to more applied reset current during off-time, and theshallower B-H curve (solid line) corresponds to less applied resetcurrent during off-time. Application of the larger amount of resetcurrent corresponding to the dotted line in FIG. 10 corresponds to thethird cycle in FIGS. 8 and 9. Application of the smaller amount of resetcurrent corresponding to the solid line in FIG. 10 corresponds to thesecond cycle in FIGS. 8 and 9. Current I_(L) only passes through themagnetic amplifier during on-time and when the magnetic amplifier is insaturation, which corresponds to the nearly horizontally section of theB-H curve. The amount of current I_(L) that passes through the magneticamplifier can therefore be adjusted by controlling the amount of resetcurrent applied to the magnetic amplifier, which controls the amount oftime the magnetic amplifier is saturated during on-time. It isunderstood that the waveforms shown in FIGS. 5-9 are for exemplarypurposes only and that other waveforms can be used and achieved.

Referring to the power circuit of FIG. 3, each magnetic amplifier isused to independently control the voltage supplied to each of the LEDstrings. The control circuit 122 measures the voltage across the currentsink transistor Q2, by measuring the voltage at point A, and alsodetermines the LED string current passing through the resistor R1 bymeasuring the voltage at point B. These measurements are then used togenerate a control signal for the reset current source to supply aproper amount of reset current to the magnetic amplifier LM1. The properamount of reset current enables a specified amount of voltage at pointA, the minimum compliance voltage, that results in a minimum amount ofcurrent loss in the current sink transistor Q2. In some embodiments, thedefined brightness of each LED string is the same for all LED strings inthe power circuit. As applied to FIG. 3, the transistor Q1 functions toprovide the reset current to the magnetic amplifier.

Use of a magnetic amplifier provides many advantages, especially whencompared to MOSFETs used in this type of application. First, magneticamplifiers consisting of just a magnetic core and a winding can operateunder high voltage conditions, for example where voltage Vs reaches100-150 volts or even higher, and be very rugged. In contrast, highvoltage MOSFETs of the type used in conventional boost regulators arenot as durable and tend to breakdown more frequently. Second, switchingMOSFETs generate noise due to the switching functionality. Magneticamplifiers generate much less noise than switching MOSFETs. Third,magnetic amplifier materials are less expensive than high voltageMOSFETs. Fourth, MOSFETs require gate driver circuitry that adds to thecomplexity of the circuit. Magnetic amplifiers do not require suchdriver circuitry. Fifth, when using magnetic amplifiers, controlcircuitry coupled to the magnetic amplifier does not utilize explicitswitching elements. The reset current provided to the magnetic amplifieris DC current and naturally turns off when the diode D1 is commuted offby the reversal of the voltage at its cathode terminal.

FIG. 11 illustrates an exemplary configuration of a magnetic amplifier.The magnetic amplifier includes a core 130 and a winding 132. Terminal134 is coupled to the secondary winding of the transformer, such astransformer 104 (FIG. 3), and terminal 136 is coupled to diode D1 anddiode D2 (FIG. 3). FIG. 12 illustrates an exemplary configuration of themagnetic amplifier that has a separate control winding 138 to receivethe reset current. Using such a control winding may have benefitsbecause the winding 138 can be referenced for example to ground. Anadditional benefit is the possibility to reduce the reset current levelby way of adding more control winding turns since the reset currentfunction of the number of control winding turns. It is understood thatthe configuration shown in FIG. 12 is an exemplary configuration forconnecting a reset current source to the magnetic amplifier. It is alsocontemplated that the reset current source can be connected directly tothe winding of the magnetic amplifier, such as shown in FIG. 3. Thesingle winding 132 of FIG. 11 is used to both transmit power and toprovide the reset current. In FIG. 12, the winding 132 is used totransmit power, and the winding 138 is used to provide the resetcurrent.

FIG. 13 illustrates a conceptual block diagram of a power circuit 200according to a second embodiment. The power circuit 200 is similar tothe power circuit 100 of FIG. 3 except that the reset current source andcurrent sink for each LED string are integrated into the controlcircuit. Specifically, the control circuit 222 includes thefunctionality of the control circuit 122 as well as the reset currentsources (transistors Q1, Q3, Q5, QN) and the current sinks (transistorsQ2, Q4, Q6, QN+1) of the power circuit 100. In some embodiments, thecontrol circuit 222 is a high voltage all analog circuit having noswitching PWM control circuitry.

The power circuits are described above as including a current sinkcoupled to each LED string to regulate the current through the LEDstring. In some embodiments, the current through the LED string may bedirectly regulated by the magnetic amplifier itself, in which case thecurrent sink transistor is only used as a dimming ON-OFF switch. Onereason for using the transistor Q2 as a current sink to regulate currentis to compensate for current response time reduction resulting frompassive elements in the circuit, such as the inductor L1 and thecapacitor C1. For applications using a high refresh rate, such asdisplay devices, the pulsation of the current through the LED stringoccurs at a high rate. In such applications, the transistor Q2 functionsas a current sink that provides a means for cleanly pulsating thecurrent at high rates. For applications that do not have as high arefresh rate, the transistor Q2 may not be needed to regulate thecurrent through the LED string. In such applications, the magneticamplifier may be used to regulate the current through the LED string.The control circuit senses the current at point B (FIG. 3), and inresponse the control circuit controls the reset current supplied to themagnetic amplifier. Such a configuration can also be used for higherrate applications if and when magnetic amplifiers are capable ofhandling higher rates, for example through improvements in materialscience technology used to generate the magnetic amplifier corematerial.

The elements LM1-LMn are described above as being magnetic amplifiers. Amagnetic amplifier is an example of a device that uses the properties ofa saturable reactor. In general, each of the elements LM1-LMn can besaturable reactors that exhibit saturation characteristics similar tothose described above in relation to magnetic amplifiers. Moregenerally, the concepts can be generalized to any device that exhibitssaturation characteristics that can be manipulated to effect a dutycycle used to adjust a voltage applied to an LED string. As used herein,a magnetic amplifier refers to such a device.

The power circuit does not utilize pulse width modulation (PWM)switching controllers in the secondary side, where the secondary side isthat part of the power circuit coupled to output of the secondarywinding of the transformer 104. This includes both the power regulatingcircuitry that provides current to the LED string and the controlcircuitry that controls the reset current. The control circuit 122 isconfigured without switching elements, instead using opamp circuitry anda reference voltage. If a switching MOSFET or boost regulator were usedinstead of the magnetic amplifier, as in the conventional power circuitsshown in FIGS. 1 and 2, the control circuit would require more complexswitching circuitry to account for the on/off timing of the switchingelements.

The power circuit has been described in terms of specific embodimentsincorporating details to facilitate the understanding of the principlesof construction and operation of the power circuit. Such references,herein, to specific embodiments and details thereof are not intended tolimit the scope of the claims appended hereto. It will be apparent tothose skilled in the art that modifications can be made in theembodiments chosen for illustration without departing from the spiritand scope of the power circuit.

What is claimed is:
 1. A power circuit comprising: a magnetic amplifierhaving a power supply signal as an input, wherein the magnetic amplifiercomprises a saturable magnetic core and two windings around at least aportion of the core; a light emitting diode string coupled to an outputof the magnetic amplifier, wherein the light emitting diode stringincludes a plurality of light emitting diodes coupled in series; acontrol circuit coupled to the magnetic amplifier and the light emittingdiode string, wherein the magnetic amplifier and the control circuit areconfigured to regulate a current through the LED string; a current sinkcoupled to an output of the LED string and to the control circuit,wherein the magnetic amplifier and the control circuit are configured tomaintain a compliance voltage of the current sink; and a reset currentsource coupled to the control circuit configured to regulate the currentthrough the LED string and to control a reset current supplied to themagnetic amplifier; wherein a first winding of the magnetic amplifier isconfigured to transmit power to the light emitting diode string and asecond winding of the magnetic amplifier is configured to receive thereset current from the reset current source; wherein a B-H curve of themagnetic amplifier exhibits hysteresis where the magnetic amplifiermaintains a flux density with zero current passing through substantiallyequal to a saturation flux density level; and wherein the magneticamplifier is configured to pass a power current to the light emittingdiode string while in a saturated state and during an on-time of thepower supply signal input to the magnetic amplifier.
 2. The powercircuit of claim 1 further comprising a plurality of magnetic amplifiersand a plurality of light emitting diode strings, one magnetic amplifiercoupled to one light emitting diode string to form a magnetic amplifierand light emitting diode string pair, wherein the control circuit iscoupled to a plurality of reset current sources, one reset currentsource coupled to one magnetic amplifier and light emitting diode stringpair.
 3. The power circuit of claim 1 wherein at least one winding iscoupled to a rectifier diode.
 4. The power circuit of claim 3 furthercomprising a filter inductor coupled to the rectifier diode.
 5. Thepower circuit of claim 1 wherein the reset current source comprises atransistor.
 6. The power circuit of claim 1 wherein the current sinkcomprises a transistor.
 7. The power circuit of claim 1 wherein thecontrol circuit is configured to provide a control signal to the resetcurrent source such that the reset current source forces a reset currentthrough the magnetic amplifier during an off-time of the power supplysignal, further wherein the magnetic amplifier changes from thesaturated state to a non-saturated state due to the reset current. 8.The power circuit of claim 1 wherein the power current is delayed frompassing through the magnetic amplifier during the on-time of the powersupply signal until the magnetic amplifier changes from thenon-saturated state to the saturated state during the on-time of thepower supply signal.
 9. The power circuit of claim 8 wherein an amountof the delay is a function of an amount of the reset current.
 10. Thepower circuit of claim 1 wherein an amount of power provided to thelight emitting diode string is an inverse function of the amount of thereset current.
 11. The power circuit of claim 1 wherein the resetcurrent source, the current sink, and the control circuit are integratedon a same integrated circuit.
 12. The power circuit of claim 1 whereinthe control circuit is configured to control the current sink so as tomaintain a constant current through the light emitting diode string.